The invention relates generally to methods and materials for sequestering and storage for disposal of radioactive iodine wastes from nuclear reactor fuel cycles and nuclear legacy wastes, as well as capture and immobilization of non-radioactive iodine species.
Radioactive 129I is one of the longer-lived fission products (1.6×107 years) resulting from the generation of nuclear fuels and energy, and it is also one that is associated with considerable public concern by virtue of the mechanism whereby it may become concentrated in the human body. Historically, 129I was simply discharged to the atmosphere. Until recently, iodine is discharged to the ocean (principally the seas around Europe) for isotope dilution with the natural iodine in seawater.
With the growth of research on advanced fuel cycles in the United States and abroad, there is a strong interest in the separations and waste form development for all radioisotopes that are isolated in the developing nuclear cycles. This includes the initial trapping of gaseous iodine radioisotopes, and their incorporation into waste forms. During fuel reprocessing, the gaseous forms of radioiodine (principally I2, CH3I, HI, and HIO) must be captured in a form that is suitable for long-term storage. Whether wastes are slated for above ground storage, or underground burial, a serious need is that the radionuclides (129I & 131I) exist in highly insoluble chemical forms that will not be readily dissolved should water gain access to the site.
A second major consideration is that the wastes not exist as powders, since an accident during storage or handling could produce a cloud of radioactive dust with the potential for causing widespread contamination.
Nuclear fuel reprocessing is a technology that has been under development for more than half a century. During normal reprocessing activities, as the fuel is dissolved from the nuclear fuel rods, most of the radio-iodine is liberated and leaves as elemental iodine vapor. An international consensus has developed that incorporating radioisotopes into borosilicate glass waste forms is a convenient and acceptable (though not necessarily optimal) technology. Iodine, however, remains a notable exception, because conventional glass waste forms do not retain the iodine.
At this time, the leading technology for capturing radioiodine from the reprocessing off-gases is sorption onto a silver-loaded zeolite matrix (where the iodine reacts with silver to form silver iodide, AgI). Recent studies at Sandia indicate that the iodine is sequestered in the form of sub-micron sized silver iodide (AgI) crystals on the internal and external surfaces of zeolite particles. One of our important research findings was that if the silver is loaded to the bulk surface (as opposed to ion exchanged into the zeolite pore), much of the iodine will be trapped on the bulk surface of the zeolite crystals, with only some of it in the channels and pores of zeolite crystal. Because of surface entrapment, mild heating causes easy release of the iodine as iodine gas. Additionally, zeolites are crushable metal oxides, and can easily form powders and dust if not protected from mechanical damage.
A different approach to solving this problem is to heat the silver-loaded zeolite matrix at a temperature sufficiently high (500-700° C.), with or without pressure, to collapse the porous framework and create a densified/sintered ceramic that retains the iodine as AgI. However, the sintering temperature cannot be so high as to cause sublimation of the AgI (≈550° C.), causing subsequent release of gaseous iodine. Unfortunately, in recent tests, commercially available silver-loaded zeolites were sintered, but did not produce the expected sequestering result because too much iodine was released during processing (likely due to the surface entrapment effect).
Hence, a need exists for a highly stable binder or encapsulant material that securely sequesters particles of AgI or AgI-zeolite; and that has good mechanical strength, durability, low iodine outgassing, and low rates of leaching in groundwater.